Abstract. A photochemical box model constrained by ancillary observations was used to
simulate OH and HO2 concentrations for three days of ambient
observations during the HOxComp field campaign held in Jülich, Germany
in July 2005. Daytime OH levels observed by four instruments were fairly
well reproduced to within 33% by a base model run (Regional Atmospheric
Chemistry Mechanism with updated isoprene chemistry adapted from Master
Chemical Mechanism ver. 3.1) with high R2 values (0.72–0.97) over a
range of isoprene (0.3–2 ppb) and NO (0.1–10 ppb) mixing ratios. Daytime
HO2(*) levels, reconstructed from the base model results taking into
account the sensitivity toward speciated RO2 (organic peroxy) radicals,
as recently reported from one of the participating instruments in the
HO2 measurement mode, were 93% higher than the observations made by
the single instrument. This also indicates an overprediction of the HO2
to OH recycling. Together with the good model-measurement agreement for OH,
it implies a missing OH source in the model. Modeled OH and HO2(*)
could only be matched to the observations by addition of a strong unknown
loss process for HO2(*) that recycles OH at a high yield. Adding to the
base model, instead, the recently proposed isomerization mechanism of
isoprene peroxy radicals (Peeters and Müller, 2010) increased OH and
HO2(*) by 28% and 13% on average. Although these were still only
4% higher than the OH observations made by one of the instruments, larger
overestimations (42–70%) occurred with respect to the OH observations
made by the other three instruments. The overestimation in OH could be
diminished only when reactive alkanes (HC8) were solely introduced to the
model to explain the missing fraction of observed OH reactivity. Moreover,
the overprediction of HO2(*) became even larger than in the base case.
These analyses imply that the rates of the isomerization are not readily
supported by the ensemble of radical observations. One of the measurement
days was characterized by low isoprene concentrations (∼0.5 ppb) and
OH reactivity that was well explained by the observed species, especially
before noon. For this selected period, as opposed to the general behavior,
the model tended to underestimate HO2(*). We found that this tendency
is associated with high NOx concentrations, suggesting that some
HO2 production or regeneration processes under high NOx conditions
were being overlooked; this might require revision of ozone production
regimes.